The present invention relates to a method for cleaning of combustion gas containing impurities in a reactor plant.
The present invention provides that combustion gas to be cleaned is supplied to a lower part of a reactor vault, wherefrom it
is directed to flow substantially vertically upwards, and
to exit at the upper part of the reactor vault to an after-treatment or the like, wherein
the combustion gas is wetted with a wetting agent in the first phase of the cleaning process of the combustion gas,
in the second phase of the cleaning process of the combustion gas at least one additive reacting with impurities of the combustion gas, particularly with sulphur compounds and sulphur oxides, is supplied to the combustion gas that is wetted with a wetting agent, and
in the third phase of the process, the speed of the upwards directed flow of the combustion gas that has been treated with a wetting agent and an additive is retarded in the retardation zone of the reactor vault by enlarging the horizontal cross-section area of the reactor vault in the flow direction of the combustion gas.
A method of the above-mentioned kind is known from publication EP-121431. In the method of the publication the supply of combustion gases is carried out to the lower part of the reactor vault, whereafter wetting of combustion gases is performed and alkali used as an additive and reacting with sulphur compounds of combustion gas is added. The combustion gas exits at the upper part of the reactor vault. However, publication EP 121431 presents no disclosure in what manner it is attended to in the process and the construction of the reactor vault that the combustion gas handled with the wetting agent and the additive reacting with impurities would have a sufficient retention time in the reactor vault. A sufficient retention time ensures for the first that the wetting of the combustion gas will be succeeded. Second, mixing of the wetted combustion gas and the additive as well as chemical reactions require a certain time. Further, sufficient drying of exiting combustion gases (removal of wetting agent drops) should be secured before the combustion gas is removed from the reactor vault.
It is an aim of the present invention to present a method by which it is possible to a great extent to eliminate the drawbacks of prior art in a manner that the impurities of the combustion gas exiting the reactor vault of the reactor plant, particularly sulphur oxides that have reacted with water in connection with wetting, have reacted with the additive, and the combustion gas is free from wetting agent drops to the extent that it is possible to avoid harmful chemical additional reactions from taking place in the after-treatment of combustion gases, particularly in a filter. For attaining this purpose, the method of the present invention is primarily characterised in that
in the fourth phase of the cleaning process of the combustion gases, the average flowing speed of the combustion gas flow is standardised to correspond substantially to the end speed of the combustion gas flow of the third phase of the process by forming in the reactor vault a retention zone having a substantially standardised horizontal cross section after the retardation zone to form an extension to the same, and that
the combustion gases are removed from the reactor vault after the retention zone.
By using the above-described solution, advantageously in combination with a filter, preferably a fibre filter, a method is obtained by which it is possible to economically resolve simultaneous removal of sulphur compounds and particles particularly, but not exclusively, in small and medium sized combustion plants (1 to 50 MW). Treating combustion gas containing sulphur oxides (SOx) with alkali compounds, particularly with calcium hydroxides is a generally known method for binding harmful sulphur to an alkali compound. However, the reactions require that the sulphur oxides and alkali oxides, in particular calcium hydroxide, are in an aqueous solution. The structure of the reactor vault has to be designed in a manner that there is sufficient time also for these reactions to take place. On the other hand, it is not sensible to discharge combustion gas containing wetting agent drops from the reactor vault, since it is advantageous in combustion plants to add to the treatment of combustion gas a filter positioned after the reactor plant and intended for dust removal, in which filter the occurrence of acidic sulphur compounds dissolved in water is very harmful. The method according to the present invention resolves the above-described problem complex in a manner that in the retardation zone which enlarges upwards in the flow direction of the combustion gases brings about a sufficient retention time for the combustion gas and thus, at the same time, a sufficient reaction time, since with an appropriate selection for the drop size the retention of the wetting agent drops is longer than the retention of the combustion gas. The retention zone forming an extension to the retardation zone ensures that the combustion gases leaving the upper part of the reactor vault do not contain wetting agent drops, so harmful sulphur compounds dissolved in water exist neither after the reactor vault in the combustion gas nor thus in the filter intended for dust removal of the combustion gas.
The accompanying dependent claims describe some advantageous embodiments of the method in accordance with the present invention.